1. Field of the Invention
The present invention relates to a control apparatus for tracing a weld line in a welding apparatus and a control method therefor, more particularly, to a control apparatus for enabling a welding torch to trace a weld line while oscillating the welding torch by detecting an electric signal responsive to changes in an arc length and a wire extension, and a control method therefor.
2. Description of Related Art
Conventionally, there has been widely used a control method for enabling a welding torch to trace a welding line according to a detection signal of an arc sensor for detecting an electric change responsive to changes in the arc length and the wire extension caused when the welding torch is oscillated.
The principle of control methods for tracing the weld line using the arc sensor which have been used in the conventional arc welding methods is applicable to the spray arc down welding method for works of a relative thick plate having a thickness larger than 2.0 mms.
The conventional arc sensor of this type has been developed on basis of the aforementioned principle, and can yield relatively good results in the horizontal fillet welding provided that the welding speed is relatively low, for instance, of an order of 120 cms/min. Also, in the horizontal fillet welding, the arc sensor is applicable to a thin plate of an order of 2.0 mms in thickness.
However, in the case of a thin plate welding for various kinds of parts of automobiles or a vertical down welding for various kinds of works, there has been widely used the short arc welding method utilizing a short circuit phenomenon for welding works in a higher welding speed with a shorter arc length in order to limit the heat input. Further, there has been widely used the MAG pulse arc welding method for welding works with a shorter arc length by sometimes producing a short circuit in order to prevent from producing the spatter in the welding utilizing the above short arc welding method.
However, the conventional arc sensor is applicable to control methods for tracing a weld line by detecting a change in the welding current due to a change in the extension length of the end of the wire and performing various kinds of calculations under the condition of an approximately constant of the arc length upon producing the arc in a welding utilizing the spray arc welding method as described above.
On the other hand, the conventional arc sensor is not applicable to the above short arc welding method for limiting the heat input and the MAG pulse arc welding method since the welding current due to the short circuit changes considerably because of the shorter arc length.
In the conventional arc welding, when there changes the thickness and the material of the works to be welded and the welding position, it is necessary to change the welding current, the welding voltage and to alter the kind of the shielding gas so as to change the arc length and the amount of the heat input, and also it is necessary to select the best welding method among the spray arc welding method, the MAG pulse arc welding method wherein the short circuit phenomenon occurs relatively rarely, and the short arc welding method wherein the short circuit phenomenon often occurs. Recently, one robot is used in order to enlarge the application range, namely, to apply to various kinds of welding method, particularly the short arc welding method.
However, when the conventional arc sensor control method is applied to the short arc welding method, it is very difficult to set a suitable frequency for detecting the arc current as follows:
In an arc phenomenon produced in the short arc welding method wherein the productions of the arc and the short circuit are repeated alternately, the continuous time interval of the short circuit phenomenon is relatively short such as several msec., and the frequency thereof is several hundreds Hz. On the other hand, the frequency of the change in the welding current to be detected by the arc sensor is about 20 Hz even though a higher oscillation frequency is set. Therefore, in the conventional method, after the detected analogue welding current is passed through an analogue low pass filter having a cut-off frequency of about 50 Hz, the analogue signal having passed therethrough is sampled at a sampling frequency in the range from 200 Hz to 500 Hz, and then, an electric signal having a frequency of about 20 Hz is outputted as a control signal for tracing a weld line.
In this conventional method, the analogue signal having passed through the analogue low pass filter has a phase delayed by a phase of about 30.degree. from a welding torch position signal, and it is extremely difficult to compensate an error due to the above phase delay including the dispersion of characteristics of the above filter highly precisely by changing the phase delay depending on changes in the processing conditions of various kinds of short arc welding. Accordingly, the conventional arc sensor control method can not be applied to the short arc welding method.
Namely, it is necessary to set a suitable oscillation frequency in the range from 0.5 Hz to 10 Hz depending on respective welding conditions such as the thickness of plates to be welded. However, it is extremely difficult to compensate the error due to the above phase delay highly precisely by setting a suitable oscillation frequency when respective welding conditions are changed widely.
Therefore, in the control method for tracing the weld line utilizing the conventional arc sensor which is applicable to only the above-mentioned short arc welding method, there is such a problem that it is extremely difficult to more widely set the welding conditions for the same welding apparatus.